1. Field of the Invention
The present invention relates generally to a method and apparatus useful in a coal mining operation to monitor the thickness of a mine roof-supporting coal rib which separates adjacent entries cut into a coal seam and, more particularly, to a method and apparatus for monitoring the thickness of a coal rib during rib formation so that the thickness of the coal rib may be controlled.
2. Description of the Prior Art
In coal mining operations, and particularly in highwall mining operations, coal is removed from a coal seam by a highwall mining machine which utilized a cutting head located at the front end of the machine to dislodge coal from the seam as the mining machine advances into the seam. The dislodged coal is transferred from the entry by a continuous haulage conveyor system to a storage location for subsequent removal from the coal mine. As can be appreciated, the entire coal seam cannot be removed by the mining machine for a variety of reasons. One primary reason is that removal of the entire coal seam from its underground location will result in the removal of all support for the earthen "roof" or overburden above the coal seam. Obviously, with all support for the mine roof removed, mine personnel working in and around the coal seam would be placed in great jeopardy.
Since the entire coal seam cannot be removed from its underground location, coal is removed from the seam by operation of the highwall mining machine to cut a series of "entries" in the coal seam. In order to provide support for the mine roof during the process of removing coal from the coal seam, each newly cut entry in the seam is separated from the previously cut entry by a selected distance so that a "rib" of coal remains after the newly cut entry is completed. The coal rib extends from the mine floor to the mine roof, and the series of coal ribs formed as multiple entries are cut in the coal seam act as roof support pillars.
Since coal ribs play such a critical role in providing a safe environment for personnel working in and around the coal seam, mine operators have been searching for a cost-effective method and apparatus for maintaining a constant coal rib thickness so that their equipment will not punch through a previously cut, adjacent entry during a new entry-cutting operation and become stuck between adjacent entries. In addition, operators have been searching for a way to maintain a constant coal rib thickness since if a coal rib is too thin it will not provide adequate support for the roof above the coal seam, and, conversely, if the rib is too wide, those coal resources associated with the coal rib become permanently sterilized (rendered inaccessible and unrecoverable).
Presently, highwall mine operators do not have a convenient, cost-effective method for monitoring coal rib thickness. One way operators now monitor coal rib thickness is to drill horizontal holes through the rib, which requires that the roof adjacent to the rib first be supported so that a drilling operator can perform the drilling operation. This method is very costly and time-consuming since the mining machine must be stopped in order to allow the measurement hole to be drilled and the thickness measurement made. More importantly, the drilling operation is unsafe since it requires that the drilling operator be placed in close proximity to the highwall mining machine and directly under a portion of the mine roof that is only temporarily supported. In order to provide some reasonable assurance that the thicknesses of coal ribs between adjacent mine entries are sufficient to provide adequate roof support, current practice requires that accurate survey measurements of the coal seam be made and that the highwall mining machine be aligned properly with respect to these survey measurements. Although these precise survey measurements are presently routinely made, they are only of limited value due to current technology limitations which limit the depth of penetration of a cut into the coal seam. For example, directional inaccuracies in the movement of the highwall mining machine may cause adjacent entries to intersect resulting in an area of unsupported mine roof which may collapse onto the highwall mining machine. In addition, both the coal geology and the operating characteristics of the mining machine will affect how well the mining machine will track the previous cut.
As can be seen from the foregoing, although equipment and methods exist for monitoring the thickness of a coal rib formed between adjacent entries cut in a coal seam they are not without their shortcomings. Consequently, there is a need for an improved apparatus and method for monitoring the thickness of a coal rib which does not require the mining operation to be suspended in order for the measurement to be made and eliminates the safety hazards associated with presently known and utilized equipment and methods. The improved method and apparatus for implementing the method should be capable of monitoring the thickness of a coal rib during rib formation so that the operation of the mining machine may be adjusted on a real time basis to instantaneously control the thickness of the coal rib to any desired thickness.